Semiconductor processing includes deposition processes such as chemical vapor deposition (CVD) of metal, dielectric and semiconducting materials, etching of such layers, ashing of photoresist masking layers, etc. Such semiconductor processes are typically carried out in vacuum chambers wherein process gas is used to treat a substrate such as a semiconductor wafer, flat panel display substrate, etc. The process gas can be supplied to the interior of the vacuum chamber by a gas distribution system such as a showerhead, a gas distribution ring, gas injectors, etc. Reactors having plural gas distribution systems are disclosed in U.S. Pat. Nos. 5,134,965; 5,415,728; 5,522,934; 5,614,055; 5,772,771; 6,013,155; and 6,042,687.
In the case of etching, plasma etching is conventionally used to etch metal, dielectric and semiconducting materials. A plasma etch reactor typically includes a pedestal supporting the silicon wafer on a bottom electrode, an energy source which energizes process gas into a plasma state, and a process gas source supplying process gas to the chamber.
A common requirement in integrated circuit fabrication is the etching of openings such as contacts and vias in dielectric materials. The dielectric materials include doped silicon oxide such as fluorinated silicon oxide (FSG), undoped silicon oxide such as silicon dioxide, silicate glasses such as boron phosphate silicate glass (BPSG) and phosphate silicate glass (PSG), doped or undoped thermally grown silicon oxide, doped or undoped TEOS deposited silicon oxide, etc. The dielectric dopants include boron, phosphorus and/or arsenic. The dielectric can overlie a conductive or semiconductive layer such as polycrystalline silicon, metals such as aluminum, copper, titanium, tungsten, molybdenum or alloys thereof, nitrides such as titanium nitride, metal silicides such as titanium silicide, cobalt silicide, tungsten silicide, molybdenum silicide, etc. A plasma etching technique, wherein a parallel plate plasma reactor is used for etching openings in silicon oxide, is disclosed in U.S. Pat. No. 5,013,398.
U.S. Pat. No. 5,736,457 describes single and dual “damascene” metallization processes. In the “single damascene” approach, vias and conductors are formed in separate steps wherein a metallization pattern for either conductors or vias is etched into a dielectric layer, a metal layer is filled into the etched grooves or via holes in the dielectric layer, and the excess metal is removed by chemical mechanical planarization (CMP) or by an etch back process. In the “dual damascene” approach, the metallization patterns for the vias and conductors are etched in a dielectric layer and the etched grooves and via openings are filled with metal in a single metal filling and excess metal removal process.
It is desirable to evenly distribute the plasma over the surface of the wafer in order to obtain uniform etching rates over the entire surface of the wafer. Some gas distribution chamber designs include multiple supply lines and multiple mass flow controllers (MFCs) feeding separate regions in the chamber. However, these gas distribution designs require numerous components, complexity in design and high cost. It therefore would be desirable to reduce the complexity and cost to manufacture such gas distribution arrangements.
U.S. Pat. No. 6,333,272, which is incorporated by reference, describes a dual feed gas distribution system for semiconductor processing, wherein a processing chamber 10 is supplied processing gas through gas supply line 12 (which can provide process gas to a showerhead or other gas supply arrangement arranged in the upper portion of the chamber) and a gas supply line 14 (which supplies processing gas to a lower portion of the chamber such as, for example, to a gas distribution ring surrounding the substrate holder or through gas outlets arranged in the substrate support), as shown in FIG. 1. However, an alternative dual gas feed arrangement can supply gas to the top center and top perimeter of the chamber. Processing gas is supplied to the gas lines 12, 14 from gas supplies 16, 18, 20, the process gasses from supplies 16, 18, 20 being supplied to mass flow controllers 22, 24, 26, respectively. The mass flow controllers 22, 24, 26 supply the process gasses to a mixing manifold 28 after which the mixed gas is directed to the flow lines 12, 14. Flow line 12 may include a combination of a flow meter 42 and a feedback controlled throttling valve 44 and flow line 14 may include a flow measurement device 34 and a feedback control valve 36, so that the process feed gas is split using two throttling valves and two flow meters. A control system 40 monitors the flow measurement devices 34 and 42 and is effective to control the mass flow controllers 22, 24, 26 as well as the feedback control valves 36 and 44. This feedback control system allows adjustment of the proportion of mixed gas delivered to two zones of the processing chamber. The open aperture of one or both of the throttle valves can be adjusted based upon a comparison of the user selected flow-splitting and flow meter readings. Conveniently, the combination of the flow meter and throttling valve could be implemented using a conventional mass flow controller, where the control system sends separate flow setpoint controls to each leg to achieve the user's selected flow splitting.
In operation, the user would select set points for the flows of each feed gas within the gas box, and would select the fraction of mixed flow to be delivered to each region of the processing chamber. For example, the user might select a flow of 100 sccm Cl2/200 sccm BCl3/4 sccm O2 with 75% delivered through line 12 and 25% through line 14. The fraction of mixed flow in the respective delivery lines is controlled by repeated adjustment of the feedback control valve 36 in line 14 based upon the actual flow measured in line 14 with respect to its target flow, while the feedback control valve 44 in line 12 is full open. By comparing the total flow, which in this case could be measured by summing all of the flow readouts of the mass flow controllers 22, 24, 26 in the gas box, with the flow measured by the meter 42 in the chamber delivery line 12, the controller can adjust the degree of throttling in the valve 36 in line 14 to achieve the desired flow distribution. Alternatively, an optional total flow meter could be installed just downstream of the mixing manifold 28 to measure the total flow of mixed gas, rather than determining the total flow by summing the readouts of the MFCs 22, 24, 26 in the gas box.
In the case where the total flow is determined by summing the gas box MFC readouts, these measured flow rates can be converted to equivalent standard cubic centimeters per minute (sccms) of a reference gas, such as nitrogen, to provide accurate and flexible control in the general case where the gas mixture may differ from process to process. Hence, a calculation could be performed to convert mixed gas flow to a “nitrogen equivalent flow” and the in-line flow measurement device in line 14 could be calibrated to measure “nitrogen equivalent flow” to put all flow measurements on the same basis. As an example, in a typical thermal-based mass flow meter 100 sccm of Cl2 is equivalent to 116.5 sccm of nitrogen, 200 sccm of BCl3 is equivalent to 444.4 sccm of nitrogen, and 4 sccm of O2 is equivalent to 4.08 sccm of nitrogen. Hence, the “nitrogen equivalent flow” of the mixed gas in the example above is 564.98 sccm and to deliver 25% through the line with the feedback controlled valve, the control loop could adjust the valve to achieve a flow reading of 0.25*564.98=141.2 sccm of nitrogen for this example. Note that at steady state, the entire flow of mixed gas from the gas box will ultimately reach the chamber, because the optional flow restrictor in line 12 is not being adjusted during the process, and the pressure will naturally build in the mixing manifold until the total in flow equals the total out flow.